1. Field of the Invention
The present invention relates to a power supply device, a fixing device and an image forming apparatus, each of which has a voltage resonance circuit including an output coil for boosting an input DC (direct-current) voltage to a predetermined voltage and outputting the boosted voltage to the load and including a capacitor connected to the output coil.
2. Description of the Related Art
Power supply devices having a conventional voltage resonance circuit include electromagnetic induction heating (EMIH) power supply devices in which eddy currents are generated in the load by electromagnetic induction using an output coil of the voltage resonance circuit as a heating coil, whereby the load itself is made to generate heat.
FIG. 1 is a circuit diagram of a conventional EMIH power supply device.
An EMIH power supply device 10 includes a rectifier circuit 20, an inverter circuit 30, a power control circuit 40 and a drive circuit 50.
The rectifier circuit 20 eliminates the power-supply noise on an AC voltage supplied by a commercial power supply (AC voltage: 100 V), and rectifies the AC voltage to a DC voltage. The rectifier circuit 20 then smoothes the DC voltage before supplying it to the inverter circuit 30.
The inverter circuit 30 includes a heating coil L1, a resonance capacitor C1 and a switching element 11, and converts a voltage supplied from the rectifier circuit 20 into a high-frequency pseudo-voltage by switching of the switching element 11. In the inverter circuit 30, electric current flows into the heating coil L1 when the switching element 11 is ON, and voltage is applied to the resonance capacitor C1 when the switching element 11 is OFF. The EMIH power supply device 10 induces eddy currents in a load 60 positioned close to the heating coil L1 by passing an electric current through the heating coil L1, to thereby heat the load 60. Note that the load 60 is made of metal and is a heating element that is heated due to eddy currents. A metal pan is a specific example of the load 60.
The power control circuit 40 detects a zero-cross point of the high-frequency pseudo-voltage converted by the inverter circuit 30, and controls the switching element 11 to turn ON/OFF at the detected zero-cross point. The power control circuit 40 is connected to a main apparatus control circuit 70 that controls a main apparatus on which the EMIH power supply device 10 is mounted. The main apparatus control circuit 70 detects the temperature of the load 60 using a temperature sensor 80 provided near the load 60. Based on the detected temperature, the main apparatus control circuit 70 outputs to the power control circuit 40 a control signal for controlling the switching element 11 so as to adjust the temperature of the load 60 to a desired value.
Accordingly, the power control circuit 40 controls the temperature of the load 60 based on the control signal from the main apparatus control circuit 70 while reducing switching losses by performing switching operations at the zero-cross point of the high-frequency pseudo-voltage. The drive circuit 50 operates the switching element 11 based on a control signal from the power control circuit 40.
Technology relating to such EMIH power supply devices is presented in Patent Document 1, for example. Patent Document 1 discloses an induction heating method, an induction heating device, a fixing device and an image forming apparatus, in each of which chopping control is performed on DC, after being rectified from AC, by the repetition of a switching element being turned ON and OFF and then the chopped DC is supplied to a resonance circuit that includes an electric coil positioned close to a heating object and also includes a resonance capacitor connected to the electric coil.
Patent Document 1: Japanese Laid-open Patent Application Publication No. 2002-237377
However, the above-mentioned conventional EMIH power supply devices need to internally have the power control circuit 40 for controlling switching of the switching element 11. The power control circuit 40 is usually realized by a microcomputer or the like, and thus high in cost.
In the case where the temperature of a load, which is an object (target) of the temperature control, changes rapidly, such control using a microcomputer requires complex controls to perform switching operations in accordance with the rapid changes in temperature. It is, therefore, expected that realizing proper temperature control of the load leads to a further increase in costs.